This invention relates to an epoxy resin composition which cures under optimum reaction conditions to B-stage, giving a semi-cured product having a minimized variation, a laminate film using the semi-cured product and suited for semiconductor packaging, and a semiconductor device sealed with the film.
The recent trend of electronic equipment is characterized by high speeds of size and weight reduction. Under the circumstances, it is desired to reduce the size and weight of semiconductor devices serving as the brain of electronic equipment. The ultimate target is to reduce the packaging area to the chip size, realizing a chip size package (CSP).
Epoxy resins are widely used in constructing semiconductor packages because of their good adhesion, heat resistance and moisture resistance. As the package system is diversified as mentioned above, epoxy resins are now used not only as the encapsulants well known in the art, but also as coating, die bonding and underfill materials. Most such materials are diluted with solvents or in paste form.
Of these, the solvent-diluted materials require an attention to carefully remove the solvent that volatilizes off on use, from the standpoints of the health maintenance and safety of workers, undesirably adding to the cost.
Coating materials are used in several ways. One proposed method is to cover the surface of a semiconductor chip solely with a thin film of an epoxy resin composition to effect sealing. It is also proposed to screen print an epoxy resin composition in paste form. These materials include paste-like epoxy resin compositions such as those of the acid anhydride curing type and the amine curing type which are well known in the art. Alternatively, these paste-like epoxy resin compositions may take the form of a film which has been converted to B-stage on a support.
Most of the prior art die bonding and underfill materials were paste-like epoxy resin compositions. Recently, films formed from paste-like die bonding materials are widely utilized. On use, a film is pressed against a lead frame or substrate, and a semiconductor chip is joined onto the film whereupon the film is cured. Most of these die bonding materials are epoxy resin compositions of the acid anhydride curing type and the amine curing type which are well known in the art.
These B-staged films are easier to handle than paste-like materials and provide a very promising system that contributes to the simplification of a semiconductor device fabricating process. However, the epoxy resin compositions of the acid anhydride curing type and the amine curing type are insufficient in the storage of uncured compositions and the moisture resistance and high-temperature performance of cured compositions. They are unsatisfactory in directly covering the semiconductor chip surface with a thin film. In regard to the reaction of converting to B-stage the epoxy resin compositions of the acid anhydride curing type or the amine curing type, it is difficult to optimize the reaction conditions to B-stage to produce a semi-cured product having a minimized variation because the straight chain growth reaction and the crosslinking reaction of the epoxy resin are identical.
Therefore, an object of the invention is to provide an epoxy resin composition which uses two curing agents separately for straight chain growth reaction and crosslinking reaction of an epoxy resin to optimize the reaction conditions to B-stage, giving a semi-cured film having a minimized variation.
Another object of the invention is to provide a laminate film comprising a flexible, easy-to-work thin layer of the epoxy resin composition in a semi-cured state.
A further object of the invention is to provide a semiconductor device in which the gap between a semiconductor chip and a substrate or the surface of a semiconductor chip is sealed with the epoxy resin composition in film form.
We have found that by using two distinct curing agents corresponding to straight chain growth reaction and crosslinking reaction (reaction to form a three-dimensional structure) of an epoxy resin, the reaction conditions under which the epoxy resin composition is converted into B-stage are optimized so that a semi-cured product may be obtained at a minimized variation. This is especially true when a difunctional epoxy resin is used as the epoxy resin and a difunctional phenolic resin is used as the curing agent for straight chain growth reaction. Further, when an imidazole compound is used as the curing agent for crosslinking reaction, and when an organic phosphorus compound is further blended as a curing accelerator for the B-staging reaction between components (A) and (B) to extend the chain length, the storage stability of a semi-cured product and the moisture resistance of a cured product are improved over prior art epoxy resin compositions of the acid anhydride curing type or the amine curing type. The semi-cured or B-staged product of the epoxy resin composition gives a film which is flexible and easy to handle. When the surface of a semiconductor chip or the gap between a semiconductor chip and a substrate is coated or sealed with this film, there is obtained a semiconductor package having improved reliability.
In a first aspect, the invention provides an epoxy resin composition comprising (A) an epoxy resin, (B) a first during agent for polymerizing the epoxy resin (A) into a linear polymer, and (C) a second curing agent for crosslinking the linear polymer of components (A) and (B) into a three-dimensional polymer. An epoxy resin composition comprising a linear polymer obtained by previously reacting components (A) and (B) before the start of component (C) to form a three-dimensional crosslinked structure, and component (C) is also provided as a semi-cured or B-staged product. In one preferred embodiment, component (A) is a difunctional epoxy resin, component (B) is a difunctional phenolic resin, component (C) is an imidazole compound. Further preferably, an organic phosphorus compound is blended as a curing accelerator for components (A) and (B).
In a second aspect, the invention provides a laminate film comprising at least two layers, at least one of which is formed of the epoxy resin composition in a semi-cured state to a thickness of 20 to 150 xcexcm.
In a third aspect, the invention provides a semiconductor device in which a surface of a semiconductor chip or a gap between a semiconductor chip and a substrate is sealed with the laminate film.